The trend in semiconductor device processing is to make the devices smaller so that more devices can be fabricated in a given area. This scale down affects substantially all of the device, so that each feature is scaled down. This is particularly problematic for the gate structure and capacitors, because capacitance is proportional to the dielectric constant of the material situated between the two plates of the capacitor and effective area of the dielectric material. In addition, the capacitance of a structure is inversely proportional to the distance between the two electrodes of the structure. Currently, since SiO.sub.2 is the material of choice for gate dielectrics, the thickness of this layer is decreased to compensate for the scaling down of the area of the capacitor. However, this thinning of the oxide layer is becoming problematic for a couple of reasons. First, as the thickness of the silicon dioxide layer is decreased to below about 3 nm, the leakage through the oxide becomes unacceptably high. In addition, the oxide layer ceases to act as an effective barrier with regards to keeping dopants which are implanted into the gate electrode to increase the conductivity of the gate electrode out of the channel regions. Second, extremely thin layers, unless they are formed from a process which is self-limiting, are very difficult to reproducibly fabricate. Third, any etching away of a thin layer, especially a gate insulator, using subsequent processing to etch other structures affects the thinner layer more dramatically than it would a thicker layer because a greater percentage of the thinner layer is removed than that of a thicker layer.
Another approach to solve this problem involves changing the gate insulating material to one with a higher dielectric constant. For example, BST, PZT, TiO.sub.2 and Ta.sub.2 O.sub.5 are being considered for the next generation of gate dielectrics. However, each of these materials pose problems because the processing required to make these materials into effective gate dielectric materials conflicts with the processing of standard transistor structures. More specifically, each of these materials require a high temperature anneal in an oxygen-containing ambient, and this anneal can greatly degrade the underlying substrate and any other exposed oxidizable structures.
Hence a new material needs to be used which is relatively easy to process using standard gate structure processing techniques and which has a dielectric constant higher than that of silicon dioxide (.epsilon..apprxeq.3.9).